This invention relates to electrode materials useful in secondary lithium batteries.
Two classes of materials have been proposed as anodes for secondary lithium batteries. One class includes materials such as graphite and other forms of carbon, which are capable of intercalating lithium. While the intercalation anodes generally exhibit good cycle life and coulombic efficiency, their capacity is relatively low.
A second class includes metals that alloy with lithium metal. Although these alloy-type anodes generally exhibit higher capacities relative to intercalation-type anodes, they suffer from relatively poor cycle life and coulombic efficiency. One reason is that the alloy-type anodes undergo large volume changes during charge and discharge. This results in the deterioration of contact between the active particles and conductive diluent (e.g., carbon) particles typically combined with the active particles to form the anode. The deterioration of contact, in turn, results in diminished cycling rate behavior.
The invention provides electrode compositions suitable for use in secondary lithium batteries in which the electrode compositions have high initial capacities that are retained even after repeated cycling. The electrode compositions, and batteries incorporating these compositions, are also readily manufactured.
To achieve these objectives, the invention features an electrode composition that includes a plurality of composite particles admixed with a plurality of electrically conductive diluent particles (e.g., electrically conductive carbon particles). The composition may further include a polymeric binder (e.g., a polyvinylidene fluoride binder) in which the composite particles and diluent particles are dispersed.
The composite particles include an electrochemically active metal particle and an electrically conductive layer partially covering the metal particle. In one aspect, the layer is present in an amount no greater than about 75 wt. % of the composite particle. Preferably, the layer is present in an amount no greater than about 50 wt. % of the composite particle, and may be present in an amount no greater than about 25 wt. % of the composite particle. In a second aspect, the layer is present in an amount no greater than about 75 vol. % of the composite particle, and may be present in an amount no greater than about 50 vol. % or 25 vol. % of the composite particle. The layer improves the efficiency of the electrode by reducing the deterioration of electrical contact between the electrochemically active metal particles and the electrically conductive diluent particles during cycling.
An xe2x80x9celectrochemically active metalxe2x80x9d is a metal that reacts with lithium under conditions typically encountered during charging and discharging in a lithium battery. In contrast, an xe2x80x9celectrochemically inactive elemental metalxe2x80x9d is a metal that does not react with lithium under those conditions. In both cases, the term xe2x80x9cmetalxe2x80x9d includes materials such as silicon that are sometimes referred to as xe2x80x9cmetalloids.xe2x80x9d
An xe2x80x9celectrically conductive layerxe2x80x9d is a layer having a bulk electrical conductivity at least in the semi-conducting range, e.g., on the order of about 10xe2x88x926 ohmxe2x88x921 cmxe2x88x921 or greater.
The expression xe2x80x9cpartially coveringxe2x80x9d means that the layer, when the composite particle is contacted with an electrolyte that includes a lithium electrolyte salt, allows the electrolyte to access the underlying electrochemically active metal particle. In some cases, this involves an arrangement in which the layer is in the form of a discontinuous coating on the particle such that the underlying metal particle material is detectable using x-ray photoelectron spectroscopy (XPS). In other cases, the layer may be porous to enable the electrolyte to penetrate the layer and access the underlying metal particle. The percent porosity of the layer is determined according to the procedure set forth in the Examples, below. Preferably, the layer has a porosity on the order of about 90%.
When incorporated in a lithium battery, the electrode composition preferably exhibits (a) a specific capacity of at least about 100 mAh/ per gram of active metal for 30 full charge-discharge cycles and (b) a coulombic efficiency of at least 99% (preferably at least 99.5%, more preferably at least 99.9%) for 30 fall charge-discharge cycles when cycled to realize about 100 mAh/per gram of active metal of the composition. Preferably, this level of performance is realized for 500 cycles, more preferably for 1000 cycles.
In another preferred embodiment, the electrode composition, when incorporated in a lithium battery, exhibits (a) a specific capacity of at least about 500 mAh per gram of active metal for 30 full charge-discharge cycles and (b) a coulombic efficiency of at least 99% (preferably at least 99.5%, more preferably at least 99.9%) for 30 full charge-discharge cycles when cycled to realize about 500 mAh per gram of active metal of the composition. Preferably, this level of performance is realized for 200 cycles, more preferably for 500 cycles.
Examples of suitable metals for the electrochemically active metal particle include aluminum, silicon (e.g., amorphous silicon), tin, antimony, lead, germanium, magnesium, zinc, cadmium, bismuth, and indium. The particle may also include one or more electrochemically inactive elemental metals. Examples of suitable electrochemically inactive elemental metals include Group IB through Group VIIB elemental metals, as well as group VIII and rare earth elemental metals. Specific examples include Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, Nb, Mo, La, Hf, Ta, W, Ce, Pr, Nd, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Be, and Sm. Of this group, molybdenum, niobium, tungsten, tantalum, iron, nickel, manganese, and copper are preferred.
In one embodiment, the metal particle consists essentially of at least one electrochemically inactive elemental metal and at least one electrochemically active elemental metal in the form of an amorphous mixture at ambient temperature. For example, the mixture may consist essentially of silicon, tin, and copper. The mixture remains amorphous when the electrode composition is incorporated into a lithium battery and is cycled through at least one full charge-discharge cycle at ambient temperature. An xe2x80x9camorphousxe2x80x9d material is a material that lacks the long range atomic order characteristic of crystalline material, as observed by x-ray diffraction, transmission electron microscopy or differential scanning calorimetry.
The electrically conductive layer may take a number of forms. In one embodiment, the layer includes electrically conductive carbon particles dispersed in a polymeric binder (e.g., a polyvinylidene fluoride binder). In other embodiments, the layer is a metal layer such as a silver, copper, or chromium layer. In a particularly preferred embodiment, the composite particles feature a layer of electrically conductive carbon particles dispersed in a polyvinylidene binder partially covering the active metal particle, and the electrode composition is prepared by combining the composite particles with electrically conductive carbon diluent particles in a second polyvinylidene fluoride binder.
Lithium batteries including the above-described electrode compositions may be used as power supplies in a variety of applications. Examples include power supplies for motor vehicles, computers, power tools, and telecommunications devices.
Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof, and from the claims.